Silicon layer to improve plug filling by CVD

ABSTRACT

A method of forming an electrically conductive plug in an opening in a dielectric layer of a substrate. A layer of silicon is deposited on the walls of an opening. In one aspect, the opening is filled by depositing electrically conductive material directly over the silicon. In another aspect, the layer of silicon is exposed to a precursor gas that reacts with the silicon so as to (a) form a volatile material that consumes substantially all of the silicon and (b) deposit an electrically conductive material within the opening.

FIELD OF THE INVENTION

The invention relates generally to chemical vapor deposition (CVD)processes for forming an electrically conductive plug in an electronicsubstrate such as an integrated circuit. More specifically, theinvention relates to such a process in which a layer of silicon isdeposited before filling the plug in order to increase the homogeneityof the electrically conductive material during the filling of the plug.

BACKGROUND OF THE INVENTION

A common process sequence in manufacturing integrated circuits and otherelectronic devices is to deposit a dielectric layer over a semiconductoror metal region on a substrate, then etch a number of openings in thedielectric so that each opening exposes a contact area on thesemiconductor or metal region, then fill each opening with anelectrically conductive material so as to form a plug that makeselectrical contact with the contact area.

It is very difficult to fill an opening having a very narrow width or ahigh aspect ratio, that is, a high ratio of height to width. In such anopening, the metal or other electrically conductive material depositedto fill the opening can agglomerate while the opening is being filled,thereby preventing the metal from flowing into and filling the portionof the opening below the agglomeration. The resulting void renders theplug defective.

Conventional processes for forming a plug typically attempt to preventthe formation of voids by depositing a wetting layer or adhesion layer,typically composed of titanium nitride, on the side wall of an openingbefore depositing the metal used to fill the plug. However, we havefound that a titanium nitride wetting layer can be inadequate to preventthe formation of voids in openings having a high aspect ratio, forexample, an aspect ratio of 6 or more. This is especially true oftitanium nitride layers formed by CVD using a metallo-organic precursorgas, which is a preferred method of depositing titanium nitride when thesubstrate temperature must remain low.

Even if a narrow opening is completely filled without voids, the plugmay have undesirably high resistivity because the crystallographicstructure of the conductive material of the plug may include a largenumber of small grains rather than a small number of large grains. Alarge number of grain boundaries within the plug increases itselectrical resistance.

SUMMARY OF THE INVENTION

The invention is a method of forming an electrically conductive plug inwhich a layer of silicon is deposited on the walls of an opening beforethe opening is filled with electrically conductive material by chemicalvapor deposition (CVD). We have discovered that the silicon layerimproves the homogeneity of the material subsequently deposited to fillthe plug. The resulting plug material typically has larger grains, andhigher aspect ratio openings typically can be filled without voids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an opening in a dielectric layerof a substrate.

FIG. 2 is a schematic sectional view of an opening having abarrier/wetting layer over which a layer of silicon has been depositedaccording to the present invention.

FIG. 3 is a schematic sectional view of a plug formed according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Conventional Plug

FIG. 1 shows a conventional semiconductor workpiece or substrate 10 onwhich the processes of the invention can be performed. The substratetypically is a silicon wafer on which integrated circuits are to beformed, or a glass substrate on which electronic video display devicesand circuits are to be formed. For convenience, we use the term“semiconductor substrate” to refer to any such substrate. In all of thefollowing examples, the substrate is depicted as a silicon wafer.

The substrate includes one or more regions 12 of semiconductor orconductor material. A dielectric layer 14 overlies the semiconductor ormetal regions. The dielectric 14 is patterned with a number of openings16 so that each opening exposes an area of one of the semiconductor ormetal regions 12, this area being termed the “contact area” or “exposedarea” of the semiconductor or metal region. (We define all portions ofthe semiconductor or metal regions 12 that are not covered by thedielectric 14 as being “exposed” even though the exposed surface may becovered by thin layer of native oxide as a result of exposure of themetal to oxygen, such as oxygen in the ambient atmosphere.)

As shown in FIG. 3, each opening 16 is filled with a metal or otherconductive material 20 to form a “plug” that makes electrical contactwith the underlying semiconductor or metal region 12. The plug also iscalled either a “contact” or a “via” according to whether the underlyingregion 12 is a semiconductor region or a metal interconnect,respectively.

When the region 12 underlying the plug 20 is a semiconductor material,often a contact layer 22 is deposited directly over the semiconductorregion. The contact layer is composed of a metal whose atoms do notsubstantially diffuse into the semiconductor 12, but into which a smallamount of the semiconductor material diffuses to form a good electricalcontact. For example, when the underlying region 12 is silicon, atitanium contact layer 22 typically is deposited over the silicon.Subsequent annealing causes silicon to diffuse into the titanium to formtitanium silicide.

A diffusion barrier layer 24 typically is deposited over the contactlayer before depositing the plug material 20. The barrier layer 24prevents metal atoms of the plug material from diffusing into andcontaminating the semiconductor 12. The most commonly used material forthe diffusion barrier is titanium nitride.

To minimize agglomeration of the plug material 20 while it is beingdeposited in the opening, the side wall of each opening typically iscovered with a wetting or adhesion layer 26 composed of a materialhaving high adhesion to the plug material. When the plug material istungsten or aluminum, typical materials used for the wetting/adhesionlayer are titanium or a compound of titanium such as titanium nitride,titanium tungsten, or titanium silicide.

Because titanium nitride has both barrier properties and wettingproperties, a single layer of titanium nitride can be deposited tofunction as both the barrier layer 24 and the wetting layer 26.

Finally, the remainder of the opening is filled with a conductivematerial 20, such as tungsten or aluminum, to form the plug.

The workpiece 10 shown in FIG. 1 and the just described process stepsfor forming the described openings and layers are all conventional. Eachlayer 20-26 typically is deposited either by a sputter depositionprocess or by a chemical vapor deposition process performed in aconventional process chamber used for fabricating semiconductor orelectronic substrates. These structures and processes are described inthe following commonly-assigned U.S. patents, the content of each ofwhich is hereby incorporated by reference into the present patentspecification: U.S. Pat. No. U.S. Pat. No. 5,108,569 to Gilboa; U.S.Pat. No. 5,371,042 to Ong; U.S. Pat. No. 5,378,660 to Ngan; U.S. Pat.No. 5,443,995 to Nulman; U.S. Pat. No. 5,525,543 to Chen; U.S. Pat. No.5,911,113 to Yao; and U.S. Pat. No. 5,943,600 to Ngan.

Metal CVD over Silicon Layer

As stated above in the Background of the Invention, when the opening 16has a very high aspect ratio, it is difficult to deposit the metal orother conductive material 20 so as to fill the opening without formingvoids, even when the side wall of the opening is covered with a titaniumnitride adhesion layer 26.

We found that void-free plugs were especially difficult to form when thetitanium nitride was deposited by a metallo-organic chemical vapordeposition (MOCVD) process, i.e., deposition by decomposition of atitanium-containing organic compound. Conventional MOCVD processes fordepositing titanium nitride are described in U.S. Pat. No. 5,723,382 toSandhu et al.; U.S. Pat. No. 5,246,881 to Sandhu et al.; and in theabove-referenced U.S. Pat. No. 5,943,600 to Ngan, the entire content ofeach of which is hereby incorporated by reference into the presentpatent specification.

MOCVD processes often are desirable for depositing the titanium nitridebecause they can be performed at a lower substrate temperature thanprocesses using an inorganic titanium precursor gas such as TiCl₄. Also,compared to sputter deposition, MOCVD typically provides more uniformside wall coverage of high aspect ratio openings. However, titaniumnitride films produced by MOCVD often have a high content of oxygen,carbon, and possibly other constituents of the organic process gasesused in the MOCVD process, and we believe that such impurities in thetitanium nitride can impede nucleation of metal film deposited in asubsequent metal CVD process. For example, we observed voids whenattempting to employ a conventional tungsten CVD process to fillopenings having an aspect ratio of 6, where the sides and bottoms of theopenings were covered with titanium nitride deposited by MOCVD.

We discovered that such openings could be successfully filled withoutvoids by depositing a layer of silicon 30 over the titanium nitridebarrier layer 24 and wetting/adhesion layer 26 as shown in FIG. 2, andthen filling the opening with conductive material 20 by conventionalCVD. We discovered that the silicon layer promotes the formation of acontinuous nucleation layer rather than discrete “islands” of isolatednucleation sites during initial deposition of the conductive material,which results in the deposition of a continuous, smooth, homogeneouslayer of the conductive material that appears to have very few grainboundaries.

One advantage of our process is that the conductive material deposits ina continuous film over the side wall of the opening, thereby avoidingagglomeration of the conductive material that can produce voids in theplug. A second advantage of our process is that it can produce a plughaving lower electrical resistance, this lower resistance probably beingdue to the plug material having fewer grain boundaries. A thirdadvantage is that the formation of a continuous initial nucleation layerresults in the barrier layer 24 being covered by the conductive materialmore quickly than if the initial deposition occurred in islands ofisolated nucleation sites. Consequently, the barrier layer will beexposed to process gas constituents such as fluorine for a shorter time,which may permit the use of a thinner barrier layer.

The chemical vapor deposition (CVD) process used to deposit the metal orother conductive material 20 over the silicon layer 30 can be any CVDprocess that includes a precursor gas that can react with the siliconlayer 30 to deposit the conductive material onto the walls of theopenings 16. In other words, the precursor gas can be any compound of ametal (or other conductive material) and a second constituent thatreacts with silicon. Preferably, the second constituent should reactwith silicon to form a volatile compound that can be readily evacuatedfrom the process chamber.

Although our invention is not limited by any theory of operation, webelieve the process of our invention operates as follows. Themetal-containing precursor gas reacts with the silicon atoms in layer 30so that atoms of metal from the precursor material replace the siliconatoms on the walls of the opening. In effect, the silicon layer 30functions as a template for deposition of the metal atoms from theprecursor gas. This creates a continuous, smooth, homogeneous initialnucleation layer of metal over which the remainder of the metal 20 isdeposited to fill the plug.

Our preferred process for depositing the silicon, described in detailbelow, deposits a layer of silicon 30 that is only one atomic layerdeep. Consequently, we expect all of the silicon 30 to be consumed asthe initial atomic layer of metal 20 is deposited over the silicon, sothat no silicon layer remains in the completely filled plug shown inFIG. 3.

For example, a conventional CVD process for filling an opening withtungsten 20 uses a process gas mixture containing tungsten hexafluoride(WF₆) gas and silane (SiH₄) gas. The WF₆ functions as atungsten-containing precursor, and the SiH₄ reacts with the WF₆ toproduce tungsten atoms and SiF₄ gas. If such a conventional process gasmixture is dispensed over a substrate containing openings in which alayer 30 of silicon has been deposited in accordance with our invention,the WF₆ will react with the silicon atoms in layer 30 to producetungsten atoms and SiF₄ gas. Each tungsten atom will replace a siliconatom on the walls of the openings, and the SiF₄ will be evacuated by theexhaust pump of the process chamber. After an initial atomic layer oftungsten is deposited in place of the silicon layer, tungsten depositionwill continue by reaction between the WF₆ and SiH₄ in the process gasmixture, as in a conventional tungsten CVD process.

Although our process has been described as especially useful for fillingopenings having bottom and side walls covered by titanium nitridedeposited by MOCVD, our process is not so limited. We also havesuccessfully employed our process to fill openings having bottom andside walls covered by titanium nitride 24, 26 deposited by reactivesputter deposition. Furthermore, we expect our process also would beuseful for filling openings having barrier layers or adhesion/wettinglayers composed of materials other than titanium nitride, or evenopenings not having any barrier layer or adhesion layer. In all of thesecases, we expect depositing a silicon layer 30 before performingchemical vapor deposition of the conductive material 20 to fill theopening will improve the homogeneity of the conductive material so as toprevent the formation of voids in the plug and reduce the number ofgrain boundaries in the material.

If a practitioner wanted to obtain the benefit of continuous depositionon the side wall of an opening for the primary purpose of preventing theformation of voids, and if that practitioner were willing to forgo someof the benefit of improved grain structure, then it would suffice tocover the side wall of the opening with the silicon layer 30 withoutnecessarily covering the bottom of the opening.

EXAMPLES

We demonstrated the benefits of depositing a single atomic layer ofsilicon prior to tungsten CVD as follows. We examined test wafers with ascanning electron microscope to observe the nucleation sites as thetungsten CVD process progressed. We tested flat wafers rather thanpatterned wafers, but we expect that the conditions that will improvenucleation of tungsten on a flat surface also will improve nucleation onthe side and bottom walls of a narrow opening.

To simulate a surface chemistry identical to that which typically wouldbe found on the bottom or side wall of a opening for a plug, we formedthe following successive layers on a 200 mm silicon wafer: (1) We grew a3000 Å layer of silicon oxide by annealing the silicon wafer in anoxygen atmosphere at a wafer temperature of 1000° C.; (2) We deposited200 to 300 Å of titanium by ionized metal plasma sputter deposition; (3)We deposited 100 Å of titanium nitride by an MOCVD process employingthermal decomposition of tetrakis (dimethylamide) titanium (IDMAT); (4)Some of the wafers then were exposed to a plasma to drive out theoxygen, carbon and other impurities from the titanium nitride and todensify the titanium nitride; (5) One some of the wafers we deposited asingle atomic layer of silicon by thermal decomposition of silane, usingthe silicon deposition process described below; then (6) We depositedtungsten in a thermal CVD process performed at a chamber pressure of 30Torr, with a gas mixture of the following gases and flow rates: WF₆ at30 sccm, SiH₄ at 30 sccm, Ar at 2500 sccm, and H₂ at 1000 sccm.

With the wafers for which we did not perform the plasma treatment step(4), the advantages of the invention were especially marked. With thewafers on which we deposited the silicon layer in step (5), the scanningelectron microscope photos revealed homogeneous nucleation of thetungsten after only 4 seconds of tungsten deposition time. In contrast,with the wafers on which we skipped step (5) so as to not deposit thesilicon layer, even after 7 seconds the deposited tungsten had notcoalesced. Instead, at 7 seconds we observed islands of tungstennucleation, each island being about 0.14 microns in diameter. After 10seconds, by which time the tungsten layer was 400 Å thick, the tungstenfilm finally coalesced into a continuous film.

After 15 seconds of tungsten deposition we measured the resistivity ofthe tungsten film. The resistivity was about twice as high for thetungsten that was deposited directly over the titanium nitride ascompared to the tungsten that was deposited over a mono-atomic layer ofsilicon. We believe this difference is largely due to our processproducing a tungsten film having fewer grain boundaries.

With the wafers for which we did perform the plasma treatment step (4),the difference between tungsten films deposited with and without thesilicon layer was less dramatic. However, the scanning electronmicroscope photos showed evident grains without the silicon layer,whereas the film was smooth with no evident grains when the silicon wasdeposited according to our invention.

Silicon Deposition Process

Our preferred process for depositing the silicon layer 30 is thermaldecomposition of silane. The temperature of the substrate 10 is elevatedsufficient to promote decomposition of the silane, and the substrate isexposed to an atmosphere containing silane (SiH₄) gas for a period oftime sufficient to deposit a single atomic layer of silicon on thebottom and side wall of the opening. We successfully tested this processin a conventional vacuum chamber, also called a process chamber, usedfor conventional thermal CVD processes.

We tested a range of process conditions to determine what conditionswould reliably deposit the desired uniform mono-atomic layer of silicon.For purposes of this series of tests, we used the flat test wafersdescribed above rather than patterned substrates, because we expect thatthe conditions that will successfully deposit a mono-atomic layer ofsilicon on the bottom and side walls of openings are about the same asthose that we found would deposit such a layer on a flat surface.

With the test wafers just described, we attempted to deposit amono-atomic layer of silicon by

thermal decomposition of silane. We mounted the substrate on a pedestalwhose temperature was maintained at 425° C., so that the substratetemperature was about 410° C.

The gas mixture we supplied to the chamber consisted of silane gasdiluted by argon gas. We tested chamber pressures of 30, 90 and 300Torr; silane flow rates of 30, 60, 75 and 90 sccm; and argon flow ratesof 1300 and 2500 sccm. We found that the silicon deposition process wasself-limiting, so that so after an initial mono-atomic layer wasdeposited, no additional silicon was deposited on the substrate. Wefound that a silane flow rate of at least 60 sccm reliably deposited amono-atomic layer of silicon over the MOCVD titanium nitride,independent of the chamber pressure and the argon flow rate. Flow ratesof at least 75 sccm and 90 sccm would be progressively more preferable.The time required to deposit the silicon layer typically was 10 seconds,and never required more than 60 seconds.

Conversely, we could not consistently deposit a mono-atomic layer ofsilicon with a silane flow rate of only 30 sccm, even when we reducedthe argon flow rate to 1300 sccm and increased the chamber pressure to300 Torr so that the silane partial pressure was 6.8 Torr. In contrast,one of the successful tests involved a silane flow rate of 75 sccm, anargon flow rate of 2500 sccm, and a chamber pressure of 90 Torr, so thatthe silane partial pressure was only 2.6 Torr. This suggests that asufficient silane flow rate is much more important than a sufficientsilane partial pressure in determining whether a mono-atomic layer ofsilicon can be reliably deposited.

We expect that the required silane flow rate will increase in directproportion to the surface area of the substrate to be treated. Forexample, if a 400 mm substrate were to be treated instead of the 200 mmsubstrate used in the above tests, the substrate surface area would befour times greater, so we expect the silane flow rate should beincreased by a factor of four to at least 240 sccm.

Throughout this patent specification, we use directional terms such astop, bottom, up, and down merely to mean direction relative to thesurface of a substrate or the mouth of an opening. These terms are notintended to imply any orientation relative to the direction of theearth's gravity.

What is claimed is:
 1. A method of forming a plug of electricallyconductive material in an opening in a substrate, comprising the stepsof: providing a substrate having a layer of dielectric that includes anopening laterally bounded by a side wall; depositing a layer of siliconto cover the side wall of the opening; and depositing a layer ofelectrically conductive material directly over the layer of silicon soas to fill substantially all of the opening with said layer ofelectrically conductive material.
 2. A method according to claim 1,wherein the step of depositing the layer of electrically conductivematerial further comprises: consuming the layer of silicon during thedepositing of the electrically conductive material so that substantiallynone of the silicon remains in the opening.
 3. A method according toclaim 1, wherein the layer of silicon deposited on the side wall is amono-atomic layer of silicon.
 4. A method according to claim 1, whereinthe step of depositing the electrically conductive material comprises:chemical vapor deposition of a metal using a precursor gas having afirst constituent that is said metal and having a second constituentthat reacts with said silicon to form a volatile byproduct.
 5. A methodaccording to claim 1, wherein the step of depositing the electricallyconductive material comprises: chemical vapor deposition of tungstenusing a precursor gas that includes tungsten hexafluoride.
 6. A methodaccording to claim 1, wherein the electrically conductive material istungsten.
 7. A method according to claim 1, further comprising the stepof: covering the side wall of the opening with a layer of titaniumnitride before depositing said layer of silicon.
 8. A method accordingto claim 7, wherein the step of covering the side wall comprises:depositing said layer of titanium nitride by metallo-organic chemicalvapor deposition.
 9. A method of forming an electrically conductive plugin an opening in a substrate, comprising the steps of: providing asubstrate having a layer of dielectric that includes an openinglaterally bounded by a side wall; depositing a layer of silicon to coverthe side wall of the opening; and exposing the layer of silicon to aprecursor gas that reacts with the silicon so as to form a volatilematerial that consumes substantially all of the silicon and so as todeposit an electrically conductive material within the opening.
 10. Amethod according to claim 9, wherein the exposing step further comprisesthe steps of: positioning the substrate within a vacuum chamber; andevacuating said volatile material from the vacuum chamber.
 11. A methodaccording to claim 9, wherein: the precursor gas comprises a compoundthat includes first and second constituents; the first constituent ofthe compound is a metal; and the second constituent of the compoundreacts with the silicon so as to form said volatile material.
 12. Amethod according to claim 9, wherein the precursor gas includes tungstenhexafluoride.
 13. method according to claim 10, wherein the electricallyconductive material is tungsten.
 14. method according to claim 10,wherein the electrically conductive material is a metal.
 15. A methodaccording to claim 10, wherein the layer of silicon comprises depositedon the side wall is a mono-atomic layer of silicon.
 16. A methodaccording to claim 10, wherein the exposing step further comprisesfilling the opening with said electrically conductive material.
 17. Amethod according to claim 10, further comprising the step of: coveringthe side wall of the opening with a layer of titanium nitride before thestep of depositing said layer of silicon.
 18. A method according toclaim 17, wherein the step of covering the side wall with said layer oftitanium nitride comprises: depositing said layer of titanium nitride bymetallo-organic chemical vapor deposition.
 19. A method of forming anelectrically conductive plug in an opening in a substrate, comprisingthe sequential steps of: providing a substrate having a layer ofdielectric that includes an opening laterally bounded by a side wall;depositing a mono-atomic layer of silicon to cover the side wall of theopening; and depositing electrically conductive material so as to fillthe opening.
 20. A method according to claim 19, wherein the step ofdepositing the mono-atomic layer of silicon comprises the steps of:positioning the substrate within a vacuum chamber, wherein the substrateis characterized by a surface area; supplying silane to the vacuumchamber at a flow rate of at least 60 sccm multiplied by the ratio ofthe surface area of the substrate to the area of a circle having adiameter of 200 mm.
 21. A method according to claim 20, wherein the stepof depositing the mono-atomic layer of silicon further comprises thestep of: during the depositing of the silicon, maintaining in the vacuumchamber a pressure in the range of 30 to 300 torr.
 22. A methodaccording to claim 19, wherein the step of depositing the electricallyconductive material comprises: chemical vapor deposition of a metalusing a precursor gas having a first constituent that is said metal andhaving a second constituent that reacts with said silicon to form avolatile byproduct.
 23. A method according to claim 19, wherein the stepof depositing the electrically conductive material comprises: chemicalvapor deposition of tungsten using a precursor gas that includestungsten hexafluoride.
 24. A method according to claim 19, furthercomprising the step of: covering the side wall of the opening with alayer of titanium nitride before depositing said mono-atomic layer ofsilicon.
 25. A method according to claim 24, wherein the step ofcovering the side wall comprises: depositing said layer of titaniumnitride by metallo-organic chemical vapor deposition.